t's what's inside those trailers, though, that is the key to Microsoft's cloud-computing efforts. Each of the shipping containers in the Chicago data center houses anywhere from 1,800 to 2,500 servers, each of which can be serving up e-mail, managing instant messages, or running applications for Microsoft's soon-to-be-launched cloud-based operating system--Windows Azure.

"I think, I'm not 100 percent sure, but I think this could be the largest data center in the world," said Arne Josefsberg, general manager of infrastructure services for Microsoft's data center operations.